Heavy marijuana use can damage short-term memory, study shows

Christopher Ingraham

New research published in the journal JAMA Internal Medicine confirms what many of us have suspected for some time: If you smoke a lot of weed - like a lot of it - it can potentially do permanent damage to your short-term memory.

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Marijuana: uncertain medicine

Marijuana's effects may vary from person to person, and scientists aren't quite sure what to make of the common distinction between different strains.

Professor Reto Auer of the University of Lausanne led a team of researchers who examined data on the marijuana habits of nearly 3400 Americans over a 25-year period. At the end of the study period, the subjects took a battery of tests designed to assess cognitive abilities - memory, focus, ability to make quick decisions, etc.

The study found that people who smoked marijuana on daily basis for a long period of time - five years or more - had poorer verbal memory in middle age than people who didn't smoke, or who smoked less. This association remained even after researchers controlled for a variety of other factors known to affect cognitive performance - such as age, education, use of other substances and depression.

The more weed people smoked, the worse they performed on the memory tests. Photo: New York Times

Auer and his team measured lifetime marijuana exposure in a fantastic new unit of measurement they call "marijuana-years". Essentially, if you smoke weed every day for a year, that equals one marijuana-year of use. Ditto if you smoke every other day for two years, or once a week for seven years.

The relationship between marijuana exposure and memory problems was essentially linear. The more weed people smoked, the worse they performed on the memory tests. But just how much worse?

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Let's say we have two groups of 10 people each. You tell each of them a list of 15 words and ask them to memorise them. Then 25 minutes later, you ask them to recall all of the words to the best of their ability.

The first group consists of 10 people who don't smoke weed or only do so occasionally. Let's say on average, people in this group would be able to remember nine out of the 15 words.

The second group consists of people who smoked marijuana every single day over a period of five years. On average, they'd be able to recall 8.5 out of the 15 words.

That doesn't seem like a huge cognitive difference, and by and large it's not. But multiply that by every five marijuana years of exposure and the gap can start to get larger. For instance, say you had a group of people who smoked weed literally every single day from age 20 until they turned 45. At age 45, you'd expect these folks to remember, on average, 2.5 fewer words as a comparable group who had smoked occasionally or not at all over the same period.

Few people actually smoke this much pot. Among the 3385 study subjects, only 311 (8 per cent) had more than five marijuana-years of exposure. But many drug policy experts are concerned that legalising marijuana and making it easier to get will cause rates of heavy, problematic use like this to rise.

One important caveat is that a study like this can't determine causality. It could be the case that heavy pot use makes your short-term memory bad, or it could be that people who operate at a lower level of cognitive function are more inclined to use marijuana heavily.

It's also worth noting that the other cognitive abilities researchers tested - focus and processing speed - did not seem to be significantly impacted by heavy marijuana use.

The association between short-term memory declines - potentially permanent ones - and heavy pot use is very real, according to this study, and shouldn't be discounted. On the other hand, it's also quite surprising that you can smoke weed literally every single day for five years, and not have it impact your problem-solving abilities or your ability to focus at all.

These findings also need to be understood in relation to what we know about the severe cognitive effects of persistent, heavy alcohol use, which include irreversible brain damage.

Overall, the take-home message is one of moderation. Whether your preferred vice is weed or alcohol or gambling or burgers, it stands to reason that if you overdo it, you're going to hurt yourself.